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2Addicts | BMW 2-Series forum Technical Topics Wheels and Tires -- Sponsored by The Tire Rack Rolling Diameter and 17 in fronts 18 in the rear fitment

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      10-28-2022, 07:31 PM   #1
almightykod
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Question Rolling Diameter and 17 in fronts 18 in the rear fitment

Hi all, This is my first post on the forums. I purchased my f22 (230i) 6 month ago, and I have been in love with that car, and this forum have tremendously helped me learn about the car, so Thanks !!!

I had an opportunity to purchase one of my favorite wheels of all time, but the downside was, the set I have purchased are 17 in the front, and 18 in the rear.

I spend plenty of time reading, and learning about how wheels work, impact of square vs staggered etc etc... and now I have 3 questions, 1 of which I cannot find an answer to anywhere.

1) If my rolling diameter is almost identical (24.72 fronts, 24.75 rears). Does this count as a square setup ? Although, the sidewalls will obviously be different (12% increase in the fronts)

2) Is running a setup like that optimal ? Are there downsides ? Aside from rotating tires etc. If it is not, What is the optimal way to run a "double staggered" setup ?

3) Does the sidewall height difference matters ?


Current plan is to run 245/40/17s in the front, and 245/35/18 in the rear. (wheels are 17x8, 18x9)


Thanks !
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      11-14-2022, 03:05 AM   #2
msej449
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1) If my rolling diameter is almost identical (24.72 fronts, 24.75 rears). Does this count as a square setup? Although, the sidewalls will obviously be different (12% increase in the fronts)

'Square' tends to mean that all dimensions are the same on all four wheels. So by common definition, no.

2) Is running a setup like that optimal? Are there downsides? Aside from rotating tires etc. If it is not, What is the optimal way to run a "double staggered" setup?

Probably not. Probably yes. What you describe may not be very desirable in terms of handling. 17"x8" fronts and 18"x9" rears are definitely not a combination ever tested by BMW, let alone approved for this model, or any of its mainstream cars. [edit: apparently not - see below]And I'm fairly sure that the feedback on here would be that this would be a very unusual and possibly dangerous setup. Or at a minimum, would produce some very unusual driving dynamics.

3) Does the sidewall height difference matters ?

Yes, significantly. All other things being equal, the lower the sidewall height, the easier the wheel is to turn but there's a price to pay - this is a zero sum game: a lower sidewall i. gives a harsher ride, ii. is less resistant to hyroplaning and iii. is more prone to pothole damage. You need to be aware of this if you swap to wider wheels/tires with narrower sidewalls (i.e. low-profile). Obviously, if you live in a low-rainfall area with good roads then you may be very happy with the result. But if you live in an area of higher rainfall with poor road surfaces you may not.

I also wonder how an insurer would treat a serious accident claim if a post-accident inspection revealed such an unusual, if not unique wheel setup. It's not like fitting a slightly wider tire, or a slightly oversized wheel with spacers. If they asked BMW, I'm fairly sure they'd be pretty negative about this as being a long way away from anything standard, to the extent of being gratuitously dangerous. Which might not bode well in terms of your liability. Sorry to sound negative, but I think that on handling dynamics alone the 17/18 setup is at best questionable, even before any of the legal issues come into the frame.

But other 2addicts may have different advice - I'd happily defer to their expertise. This is just one person's view.
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Last edited by msej449; 11-14-2022 at 03:15 AM..
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      11-14-2022, 11:23 AM   #3
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I would say if the tyre is the same make and model with the same tread compound front and rear, there shouldn't be a problem. BMW has 19/20 and 20/21" combinations front and rear on the G8x M models and this is a common configuration on Corvettes, in our track club we have this type of setup on quite a few cars with no handling issues.

Going from a square setup to staggered setup with about 10% increase in rear treadwidth compared with changing the sidewall height by 10% will likely have similar affects on the handling from an understeer/oversteer perspective, but neither should cause any drastic affect.

From an insurance perspective, most US insurers seem to be fairly blase about this type of change compared with UK insurers, from what I know.
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      11-17-2022, 09:26 PM   #4
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thanks for the great insights.

I will spend sometime looking at how g8x-m/corvette track setups differences are

I don't really know how much rolling diameter % I would need if I would to run less wall diameter, and my thinking was rolling diameter is more important than side wall difference, but I guess that might be skewed.
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      11-17-2022, 09:29 PM   #5
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Quote:
Originally Posted by msej449 View Post
1) If my rolling diameter is almost identical (24.72 fronts, 24.75 rears). Does this count as a square setup? Although, the sidewalls will obviously be different (12% increase in the fronts)


3) Does the sidewall height difference matters ?

Yes, significantly. All other things being equal, the lower the sidewall height, the easier the wheel is to turn but there's a price to pay - this is a zero sum game: a lower sidewall i. gives a harsher ride, ii. is less resistant to hyroplaning and iii. is more prone to pothole damage. You need to be aware of this if you swap to wider wheels/tires with narrower sidewalls (i.e. low-profile). Obviously, if you live in a low-rainfall area with good roads then you may be very happy with the result. But if you live in an area of higher rainfall with poor road surfaces you may not.

.
Wouldn't that mean essentially that larger wheels would have a similar impact ?
An example would be like putting 20s with a thin tire vs 17 with a large sidewall and same rolling diameter?
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      11-20-2022, 01:50 PM   #6
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I spend sometime looking at the g87 wheel tire specs and m4 wheel tire spec, and what I found was that they are focusing on diameter vs sidewall height

Example is 275/35-19 fronts, 285/30-20 rears

Which returns




The closest to a setup like that with 17/18 would be
240-40 17 Which is not an actual tire size , and the closest to that would be 235-40-17




Or even If i do my initial setup (245-40-17 with 245-35-18) I would still have a very good setup




I am planning on picking the tires up in a couple of weeks, so if there is any additional input, that would be great before I do so !

Thank you all
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      11-20-2022, 03:58 PM   #7
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I would say 245/40-17 front and 245/35-18 rear would be a suitable match, assuming the front wheel offset is optimised (relating to the tyre width not the wheel diameter), as the wrong offset and stock camber will potentially cause rubbing at the front compared with a stock 225/45-17 tyre on 7.5x17 ET43 wheel.
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